The existing PTR_UNTRUSTED branch is after the access-permission check. It only handles the case where we propagate the PTR_UNTRUSTED flag to a field pointer loaded through a BPF_READ. For a direct write such as res->key = 42, no pointer is loaded and that branch has no effect:
my_child = res->child; // in that case, my_child will be marked with PTR_UNTRUSTED, that is what the existing PTR_UNTRUSTED do. res->key = 42; // The existing check doesn't handle this. On Sun, Aug 2, 2026 at 4:32 PM Kumar Kartikeya Dwivedi <[email protected]> wrote: > > On Tue Jul 28, 2026 at 5:38 AM CEST, Ning Ding wrote: > > A kptr loaded under an RCU read lock has MEM_ALLOC set. After > > bpf_rcu_read_unlock(), the verifier marks it PTR_UNTRUSTED, but > > check_ptr_to_btf_access() still allows writes because > > type_is_ptr_alloc_obj() ignores that flag. > > > > Reject writes through MEM_ALLOC pointers that are PTR_UNTRUSTED. Reads > > remain allowed and are converted to probe-memory accesses. > > > > Also skip the owning-reference check for these pointers, since they no > > longer have a live reference after leaving the RCU critical section. > > > > Add verifier tests for the rejected write and permitted read cases. > > > > Fixes: 503e4def5414 ("bpf: Replace open code with for allocated object > > check") > > Closes: > > https://lore.kernel.org/all/[email protected]/ > > Assisted-by: Codex:gpt-5.6-sol > > Signed-off-by: Ning Ding <[email protected]> > > --- > > Same comment for this one, split kernel and selftests into separate patches. > > > kernel/bpf/verifier.c | 6 +- > > .../bpf/progs/local_kptr_stash_fail.c | 58 +++++++++++++++++++ > > 2 files changed, 62 insertions(+), 2 deletions(-) > > > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > > index 7aa47342dc65..24c151ad2e62 100644 > > --- a/kernel/bpf/verifier.c > > +++ b/kernel/bpf/verifier.c > > @@ -5804,13 +5804,15 @@ static int check_ptr_to_btf_access(struct > > bpf_verifier_env *env, > > * program allocated objects (which always have id > 0), > > * but not for untrusted PTR_TO_BTF_ID | MEM_ALLOC. > > */ > > - if (atype != BPF_READ && !type_is_ptr_alloc_obj(reg->type)) { > > + if (atype != BPF_READ && > > + (!type_is_ptr_alloc_obj(reg->type) || reg->type & > > PTR_UNTRUSTED)) { > > verbose(env, "only read is supported\n"); > > return -EACCES; > > } > > There is a PTR_UNTRUSTED check already earlier in this function, why is that > not > working or not enough? > > pw-bot: cr > > > > > if (type_is_alloc(reg->type) && > > !type_is_non_owning_ref(reg->type) && > > - !(reg->type & MEM_RCU) && !reg_is_referenced(env, reg)) { > > + !(reg->type & (MEM_RCU | PTR_UNTRUSTED)) && > > + !reg_is_referenced(env, reg)) { > > verifier_bug(env, "allocated object must have a > > referenced id"); > > return -EFAULT; > > } > > diff --git a/tools/testing/selftests/bpf/progs/local_kptr_stash_fail.c > > b/tools/testing/selftests/bpf/progs/local_kptr_stash_fail.c > > index fcf7a7567da2..d7b8cd6458bb 100644 > > --- a/tools/testing/selftests/bpf/progs/local_kptr_stash_fail.c > > +++ b/tools/testing/selftests/bpf/progs/local_kptr_stash_fail.c > > @@ -8,14 +8,22 @@ > > #include "../bpf_experimental.h" > > #include "bpf_misc.h" > > > > +extern void bpf_rcu_read_lock(void) __ksym; > > +extern void bpf_rcu_read_unlock(void) __ksym; > > + > > struct node_data { > > long key; > > long data; > > struct bpf_rb_node node; > > }; > > > > +struct plain_data { > > + long key; > > +}; > > + > > struct map_value { > > struct node_data __kptr *node; > > + struct plain_data __kptr *plain; > > }; > > > > struct node_data2 { > > @@ -31,6 +39,7 @@ struct node_data2 { > > * [35] TYPE_TAG 'kptr_ref' type_id=34 > > */ > > struct node_data *just_here_because_btf_bug; > > +struct plain_data *just_here_because_btf_bug2; > > > > struct { > > __uint(type, BPF_MAP_TYPE_ARRAY); > > @@ -82,4 +91,53 @@ long drop_rb_node_off(void *ctx) > > return 0; > > } > > > > +SEC("?tc") > > +__failure __msg("only read is supported") > > +long write_untrusted_alloc_obj(void *ctx) > > +{ > > + struct map_value *mapval; > > + struct node_data *res; > > + int idx = 0; > > + > > + mapval = bpf_map_lookup_elem(&some_nodes, &idx); > > + if (!mapval) > > + return 1; > > + > > + bpf_rcu_read_lock(); > > + res = mapval->node; > > + if (!res) { > > + bpf_rcu_read_unlock(); > > + return 2; > > + } > > + barrier_var(res); > > + bpf_rcu_read_unlock(); > > + > > + res->key = 42; > > + return 0; > > +} > > + > > +SEC("?tc") > > +__success > > +long read_untrusted_alloc_obj(void *ctx) > > +{ > > + struct map_value *mapval; > > + struct plain_data *res; > > + int idx = 0; > > + > > + mapval = bpf_map_lookup_elem(&some_nodes, &idx); > > + if (!mapval) > > + return 1; > > + > > + bpf_rcu_read_lock(); > > + res = mapval->plain; > > + if (!res) { > > + bpf_rcu_read_unlock(); > > + return 2; > > + } > > + barrier_var(res); > > + bpf_rcu_read_unlock(); > > + > > + return res->key; > > +} > > + > > char _license[] SEC("license") = "GPL"; >
